This application claims priority from EP 01304452.4 filed May 21, 2001, herein incorporated by reference.
The invention relates generally to lithographic apparatus and more particularly to reflectors.
In general, a lithographic projection apparatus comprises: a radiation system to supply a projection beam of radiation, a support structure to support patterning structure, the patterning structure serving to pattern the projection beam according to a desired pattern, a substrate table to hold a substrate, and a projection system to project the patterned beam onto a target portion of the substrate.
The term xe2x80x9cpatterning structurexe2x80x9d as here employed should be broadly interpreted as referring to structure or means that can be used to endow an incoming radiation beam with a patterned cross-section, corresponding to a pattern that is to be created in a target portion of the substrate; the term xe2x80x9clight valvexe2x80x9d can also be used in this context. Generally, the said pattern will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit or other device (see below). Examples of such patterning structure include:
A mask. The concept of a mask is well known in lithography, and it includes mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. Placement of such a mask in the radiation beam causes selective transmission (in the case of a transmissive mask) or reflection (in the case of a reflective mask) of the radiation impinging on the mask, according to the pattern on the mask. In the case of a mask, the support structure will generally be a mask table, which ensures that the mask can be held at a desired position in the incoming radiation beam, and that it can be moved relative to the beam if so desired.
A programmable mirror array. An example of such a device is a matrix-addressable surface having a viscoelastic control layer and a reflective surface. The basic principle behind such an apparatus is that (for example) addressed areas of the reflective surface reflect incident light as diffracted light, whereas unaddressed areas reflect incident light as undiffracted light. Using an appropriate filter, the said undiffracted light can be filtered out of the reflected beam, leaving only the diffracted light behind; in this manner, the beam becomes patterned according to the addressing pattern of the matrix-addressable surface. The required matrix addressing can be performed using suitable electronic means. More information on such mirror arrays can be gleaned, for example, from U.S. Pat. No. 5,296,891 and U.S. Pat. No. 5,523,193, which are incorporated herein by reference. In the case of a programmable mirror array, the said support structure may be embodied as a frame or table, for example, which may be fixed or movable as required.
A programmable LCD array. An example of such a construction is given in U.S. Pat. No. 5,229,872, which is incorporated herein by reference. As above, the support structure in this case may be embodied as a frame or table, for example, which may be fixed or movable as required.
For purposes of simplicity, the rest of this text may, at certain locations, specifically direct itself to examples involving a mask and mask table; however, the general principles discussed in such instances should be seen in the broader context of the patterning structure as hereabove set forth.
Lithographic projection apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, the patterning structure may generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising one or more dies) on a substrate (silicon wafer) that has been coated with a layer of radiation-sensitive material (resist). In general, a single wafer will contain a whole network of adjacent target portions that are successively irradiated via the projection system, one at a time. In current apparatus, employing patterning by a mask on a mask table, a distinction can be made between two different types of machine. In one type of lithographic projection apparatus, each target portion is irradiated by exposing the entire mask pattern onto the target portion at one time; such an apparatus is commonly referred to as a wafer stepper. In an alternative apparatusxe2x80x94commonly referred to as a step-and-scan apparatusxe2x80x94each target portion is irradiated by progressively scanning the mask pattern under the projection beam in a given reference direction (the xe2x80x9cscanningxe2x80x9d direction) while synchronously scanning the substrate table parallel or anti-parallel to this direction; since, in general, the projection system will have a magnification factor M (generally  less than 1), the speed V at which the substrate table is scanned will be a factor M times that at which the mask table is scanned. More information with regard to lithographic devices as here described can be gleaned, for example, from U.S. Pat. No. 6,046,792, incorporated herein by reference.
In a manufacturing process using a lithographic projection apparatus, a pattern (e.g. in a mask) is imaged onto a substrate that is at least partially covered by a layer of radiation-sensitive material (resist). Prior to this imaging step, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallization, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book xe2x80x9cMicrochip Fabrication: A Practical Guide to Semiconductor Processingxe2x80x9d, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4, incorporated herein by reference.
For the sake of simplicity, the projection system may hereinafter be referred to as the xe2x80x9clensxe2x80x9d; however, this term should be broadly interpreted as encompassing various types of projection system, including refractive optics, reflective optics, and catadioptric systems, for example. The radiation system may also include components operating according to any of these design types for directing, shaping or controlling the projection beam of radiation, and such components may also be referred to below, collectively or singularly, as a xe2x80x9clensxe2x80x9d. Further, the lithographic apparatus may be of a type having two or more substrate tables (and/or two or more mask tables). In such xe2x80x9cmultiple stagexe2x80x9d devices the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposures. Twin stage lithographic apparatus are described, for example, in U.S. Pat. No. 5,969,441 and PCT Patent Application WO 98/40791, incorporated herein by reference.
In order to meet the continual demand of manufacturers of semiconductor devices to be able to produce ever smaller features, it has been proposed to use Extreme Ultraviolet (EUV) radiation, e.g. with a wavelength of 5 to 20 nm, as the exposure radiation in a lithographic projection apparatus. Not least among the problems in designing such an apparatus is the creation of xe2x80x9copticalxe2x80x9d systems to illuminate evenly the patterning structure and to project the image of the pattern defined by the patterning structure accurately onto the substrate. Part of the difficulties in producing the necessary illumination and optical systems lies in the fact that no material suitable for making refractive optical elements at EUV wavelengths is presently known. Thus, the illumination and projection systems must be constructed out of mirrors which, at EUV wavelengths, have their own problemsxe2x80x94specifically relatively low reflectivities and extremely high sensitivity to figure errors.
It is essential in a lithographic projection apparatus that the mirrors have high reflectivities since the illumination and projection systems may have a total of eight mirrors so that, with the additional reflection at the mask, the overall transmissivity of the systems is proportional to the ninth power of the reflectivity of the mirrors. To provide mirrors of sufficiently high reflectivity, it has been proposed to use mirrors formed by multilayer stacks of materials such as Mo, Si, Rh, Ru, Rb, Cl, Sr and Be. Further details of such multilayer stacks are given in European Patent Applications EP-A-1 065 532 and EP-A-1 065 568, which documents are hereby incorporated herein by reference.
Projection systems using mirrors are particularly sensitive to figure errors at EUV wavelengths because a figure error of only 3 nm would give rise to an error in the wavefront of about xcfx80 radians, leading to destructive interference and making the reflector totally useless for imaging. Figure errors may have a variety of causes: errors in the surface of the substrate on which the multilayers are deposited, defects in the multilayers, stresses in the multilayers resulting from the manufacturing process, etc. To correct such phase errors, it is proposed in PCT Patent Application WO97/33203 to add selectively a relatively thick additional layer of crystalline or amorphous Si to the front surface of a reflector formed by a multilayer stack of Mo/Si. However, an additional layer locally reduces the reflectivity of the mirror, which may cause non-uniform illumination or exposure in lithographic projection apparatus. European Patent Application EP-0 708 367-A discloses the use of a relatively thick layer having attenuation and phase shifting functions locally deposited on a multilayer stack to form a mask pattern.
In an aspect of at least one embodiment of the present invention, there is provided improved reflectors useable with EUV radiation that have reduced figure errors and adequate reflectivity.
According to at least one embodiment of the invention, there is provided a lithographic apparatus comprising a radiation system to supply a projection beam of radiation, a support structure to support patterning structure, the patterning structure serving to pattern the projection beam according to a desired pattern, a substrate table to hold a substrate, and a projection system to project the patterned beam onto a target portion of the substrate wherein at least one of the radiation system, the projection system and the patterning structure comprises a reflector provided with a multilayer stack comprising a plurality of base periods and at least one additional period covering only part of an effective area of said reflector to effect a local change in phase and/or reflectivity relative to an adjacent area of said reflector upon reflection of said projection beam.
According to at least one embodiment of the invention there is provided a device manufacturing method comprising:
providing a substrate that is at least partially covered by a layer of radiation-sensitive material;
providing a projection beam of radiation using a radiation system;
using patterning structure to endow the projection beam with a pattern in its cross-section; and
projecting the patterned beam of radiation onto a target portion of the layer of radiation-sensitive material,
wherein said projection beam or said patterned beam is directed or patterned using a reflector provided with a multilayer stack comprising a plurality of base periods and at least one additional period covering only part of the effective area of said reflector to effect a local change in phase and/or reflectivity relative to an adjacent area of said reflector.
Further, according to at least one embodiment of the invention, there is provided a method of manufacturing a reflector for use in the radiation or illumination systems of a lithographic projection apparatus the method comprising:
providing a multilayer stack on a substrate;
determining any figure errors in the multilayer stack or substrate; and
selectively providing at least one additional period on the front surface of said multilayer stack to effect a local change in phase and/or reflectivity relative to adjacent areas of said reflection in radiation reflected by said reflector to compensate for the effects of said figure errors.
Still further, according to at least one embodiment of the invention, there is provided a phase shift mask for use in lithographic projection, said mask comprising a multilayer stack comprising a plurality of base periods and a phase-shifting mulitlayer stack comprising at least one phase-shifting period covering only part of the effective area of said mask to effect a local phase shift, different than a phase change on reflection from said base periods.
Still further, according to at least one embodiment of the invention, there is provided a device manufacturing method comprising:
providing a substrate that is at least partially covered by a layer of radiation-sensitive material;
providing a projection beam of radiation using a radiation system;
using patterning structure to endow the projection beam with a pattern in its cross-section; and
projecting the patterned beam of radiation onto a target portion of the layer of radiation-sensitive material,
wherein said using patterning structure comprises using a phase shift mask, said mask comprising a multilayer stack comprising a plurality of base periods and an additional mulitlayer stack comprising at least one additional period covering only part of the effective area of said mask, whereby said additional period effects a local change in phase and/or reflectivity relative to adjacent areas of said mask on reflection of a projection beam.
Although specific reference may be made in this text to the use of the apparatus according to at least one embodiment of the present invention in the manufacture of ICs, it should be explicitly understood that such an apparatus has many other possible applications. For example, it may be employed in the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, liquid-crystal display panels, thin-film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms xe2x80x9creticlexe2x80x9d, xe2x80x9cwaferxe2x80x9d or xe2x80x9cdiexe2x80x9d in this text should be considered as being replaced by the more general terms xe2x80x9cmaskxe2x80x9d, xe2x80x9csubstratexe2x80x9d and xe2x80x9ctarget portionxe2x80x9d, respectively.
In the present document, the terms xe2x80x9cradiationxe2x80x9d and xe2x80x9cbeamxe2x80x9d are used to encompass all types of electromagnetic radiation, including ultraviolet radiation (e.g. with a wavelength of 365, 248, 193, 157 or 126 nm) and EUV (extreme ultra-violet radiation, e.g. having a wavelength in the range 5-20 nm).